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[Display omitted] • The pH of bottom bauxite-ore from alkaline to acidic along the profile upwards. • The δ15N and redox proxies revealed that bauxite formed in the suboxic facies. • The denitrification was the predominant process at nitrogen cycling in bauxites. • The δ15N and salinity proxies revealed that higher saline favor low-oxygen. Over 5 billion tons of karstic bauxites are hosted in the Late Carboniferous Benxi Formation in the North China Craton (NCC). However, the bauxite formation environment in the central NCC remains unknown. The geological, mineralogical, geochemical, nitrogen concentrations, and δ15N isotope evidence of the Huilong and Shangwutou bauxite deposits in the central NCC was conducted, aiming to clarify the formation environments. Typically, bauxites in the central NCC contains three layers from bottom to top: Fe-bearing claystone/clayey ironstone, bauxite ore, and bauxitic claystone. The Fe-bearing claystone/clayey ironstone is composed of kaolinite, goethite, and hematite; the bauxite ore is composed of diaspore, kaolinite, but also minor hematite, ilmenite, and svanbergite; and the bauxitic claystone consists of kaolinite, but also some diaspore and goethite. Diaspores coexisted extensively with hematite, anatase, and svanbergite, forming cryptocrystalline matrix of bauxites, suggesting that they may be formed during early diagenesis. Geochemical discriminant diagrams also indicated that two deposits were primarily affected by the hydrogenous deposition and early diagenesis, and no sedimentary recirculation. The ∑REEs concentrations and fractionation (Pr n /Yb n) features along the profiles mainly concentrated in the bottom bauxite ore, which reflected that the pH values could be alkaline in bottom bauxite ore, and decreased weakly acidic for the central bauxite ore to bauxitic claystone along the profiles upwards. Combining the δ15N data with two geochemical redox proxies, and the widely coexistence of diaspore and hematite, indicated that the two deposits mainly formed under suboxic conditions, and thus denitrification was the predominant process in nitrogen cycling. The δ15N values are positively correlated with paleosalinity proxies, indicating that the higher saline bottom water may lead to a strong vertical density differential and water-column stratification, conducing to forming suboxic restricted pore-water environments. [ABSTRACT FROM AUTHOR]

>5 billion tons of karstic bauxites were formed during the Late Carboniferous in the North China Craton (NCC). However, the metallogenic mechanism of bauxite is still obscure due to the unclear formation process of the main ore mineral, diaspore. We conducted geological, mineralogical, δ34S pyrite , and in-situ diaspore trace element analyses on the Baode-Xingxian large bauxite deposit in the northern NCC, aiming to clarify the diaspore-forming mechanism and explore the genesis of bauxite. Generally, bauxites in northern NCC contains three layers from bottom to top: Fe-bearing claystone, bauxite ore, and claystone. The bauxite ore is mainly diaspore, kaolinite, and illite; the Fe-bearing claystone is illite and berthierine; and the claystone is primarily kaolinite. The broadly distributed high positive δ34S pyrite values (10‰–40‰) indicate that bauxites in the northern NCC formed in shallow-water environments. Total 216 diaspore grains have similar trace element compositions, and are obviously enriched in Cr, V, Re and U, but not in Mo, indicating that diaspore was formed in the subreduced facies. Diaspore with Fe/Al up to 4 mol% has slightly large cells, indicated that small-scale larger ions, Fe, entered the diaspore lattice to form Fe-diaspore. The favorable temperature and water activity of Fe-diaspore precipitation is similar to that of Al-hematite+ boehmite. We thus propose a probable diaspore formation mechanism: the small scale of Fe for Al substitution in the diaspore lattice and stabilize it to induce Fe-diaspore formation. Fe-diaspore is metastable with respect to diaspore and tends to convert into it during the diagenesis process. [Display omitted] • The high positive δ34S values revealed that bauxites formed in the shallow water. • The elemental proxies revealed that diaspore formed in the subreduced facies. • The Fe3+for Al3+ substitution induced to form Fe-diaspore. • Metastable Fe-diaspore tends to convert into diaspore during diagenesis. [ABSTRACT FROM AUTHOR]

The sources of karstic bauxite are complex and have not been so far addressed by multi-proxy provenance studies that interrogate bauxite sources. The Carboniferous bauxite in the North China Craton, which is the largest karstic bauxite area in the world, contains considerable quantities of detrital heavy minerals, including zircon, rutile, tourmaline, and moissanite, allowing multi-proxy provenance studies. In this study, geological, mineralogical, geochemical analyses, and detrital heavy mineral analyses were conducted for the Jiagou and Kuancaoping bauxite deposits located in the southern and northern parts of the North China Craton, respectively. Coupled with previously published geological, geochemical, and detrital mineral data, this study aims to thoroughly clarify the provenance of karstic bauxites in the North China Craton. In general, the ore-bearing rock series contains three units: lower iron-bearing claystone consisting of illite, chamosite, and goethite; middle bauxite ore dominated by diaspore, kaolinite, illite, chamosite, and anatase; and upper bauxitic claystone composed of kaolinite, diaspore, and anatase. Immobile trace element ratios and Rare Earth Element patterns of the bauxites throughout the North China Craton revealed that the lower iron-bearing claystone was autochthonous and directly sourced from the underlying limestone. The isotope data of detrital zircon along with chemical compositions and morphological features of detrital rutile, tourmaline, and moissanite in the bauxite ores revealed that the bauxite ore and bauxitic claystone were mainly para-allochthonous and derived from regional multi-sources. The multi-sources origin is also shared by other karst bauxites in the world. The results indicate that the bauxite deposits in the northern and southern parts of the North China Craton have distinct source materials. The provenance of the bauxite in the northern part of the North China Craton is mainly from the northern margin craton orogenic belt and the adjacent intraplate paleo-uplifts, whereas in the southern part, it is dominated by the North Qinling orogenic belt and the nearby paleo-uplifts. Consequently, the Carboniferous karstic bauxites in the North China Craton are characterized by the autochthonous origin of the lower iron-bearing claystone and the para-allochthonous origin of the upper bauxite ore and bauxitic claystone. This clearly illustrates the dependence of provenance information on mineral proxy choice and underlines the preponderance of multiproxy provenance studies to solve the complex source issue of karst bauxites in North China Craton. [Display omitted] • Multiproxy provenance studies of Jiagou and Kuancaoping bauxite deposits in NCC carried out • Lower iron-bearing claystone is autochthonous and directly derived from the underlying limestone. • Bauxite ore and bauxitic claystone are allochthonous and regionally multi-sourced. [ABSTRACT FROM AUTHOR]

Late Carboniferous bauxite is widely developed on the North China Craton (NCC), and is referred to as the "G" layer bauxite. The bauxite ores of the southern NCC are closely related to the tectonic evolution of the NCC and its neighboring North Qinling Belt (NQB) to the south. In this study, based on the geological and mineralogical observations, we used the U–Pb and Hf isotopes of detrital zircons and chemical compositions of detrital rutiles from the Gunziying and Shibanhe bauxite deposits, along the southern margin of the NCC to study the metallogenic process of bauxite ores and their tectonic implications. One and two bauxite ore layers were observed in the Gunziying and Shibanhe bauixte deposits, respectively. The bauxite ore is dominated by diaspore, illite, anatase, kaolinite, pyrite, zircon, and rutile. Analyses of detrital zircons revealed that the largest group (∼432–515 Ma, at a peak age of ∼450 Ma) is spread throughout the Late Carboniferous bauxite deposits, with εHf(t) values of −5.7–21.4, similar to the zircons from the NQB. Similarly, the detrital zircons with peak ages of ∼950 Ma and age clusters at ∼1.1 Ga, ∼1.3 Ga, and ∼1.8 Ga as well as those ranging in age from 635 to 797 Ma, also exhibited εHf(t) values similar to the zircons from the NQB. In addition, the results suggested that 62 (73%) and 75 (88.2%) of detrital rutiles showed metapelitic protoliths and amphibolite/eclogite facies temperatures, respectively, while 23 (27%) and 10 (11.8%) exhibited metamafic protoliths and granulite facies temperatures, respectively. Based on the results above, we propose that the provenance of the bauxite ores on the southern margin of the NCC is primarily from the NQB. The dominant contributors were the Neoproterozoic Kuanping Group and the Meso–Neoproterozoic Qinling Group. The Archean-Proterozoic metamorphic rocks in the paleo-uplifts of the NCC were also likely involved in the bauxite formation. A probable outline of the bauxite-formation process on the southern NCC, former weathered residuals and subsequent deposition, is proposed; and a reasonable interpretation that the NQB was accreted to the NCC in the Late Carboniferous and formed a northward lying slope basin within the NCC, is supported. Image 1 • New constraints on the source of the NCC bauxite ores are proposed. • Mainly material transport from the North Qinling and Kuanping Group. • Bauxitic formation most likely weathered residuals and deposited. • Late Paleozoic Tectonics resulted in the "G" bauxites throughout the NCC. [ABSTRACT FROM AUTHOR]

Above two billion tons of bauxite were formed during the exposure of the North China Craton (NCC) in 445–300 Ma. Mineralogical, geochemical, and isotopic analyses of karstic bauxite deposits in the Sanmenxia area situated within southern NCC were conducted to explore the genetic processes of Carboniferous bauxite. The bauxite contains three layers: a lower Fe-bearing claystone layer, a middle bauxite ore layer, and an upper claystone layer. Fe-bearing claystone is dominated by illite and hematite in the uplift and illite, siderite, and pyrite in the depression. Bauxite ore is composed of diaspore, illite, pyrite, and anatase, and the upper claystone consists of illite, diaspore, and anatase. Distinct pyrite δ34S values (11.7–15.2 and −11.5 to 1.0), illite (87Sr/86Sr) i (0.711917–0.713588 and 0.708000–0.711057) and ε Nd (t) (−12.2 to −11.4 and −11.8 to −9.7) values, and the immobile elemental ratios were obtained between the lower Fe-bearing claystone and upper bauxite ore and claystone. Together with chemistry of detrital rutile, it is denoted the Fe-bearing claystone was formed from the underlying limestone, whereas the bauxite ore and claystone were derived from both igneous rocks along craton margin and regional Precambrian metamorphic rocks. The visible microorganisms and wide development of framboidal pyrite with negative δ34S within the bauxite ore provide evidences of microbial activity during bauxite formation. Bio-leaching process after the deposition of allochthonous source materials caused the downward migration of ions (Al3+, Fe2+, Ti4+, REE3+, etc.). These ions encountered an alkaline and reducing condition in the karstic depression, forming diaspore, pyrite, and anatase and REE enrichment at the bottom of bauxite ore. This large input of raw materials, warm and humid climate, and microorganisms led to large-scale bauxite formation in the NCC during late Carboniferous. • Karstic bauxite in the karstic depressions in the North China Craton belongs to a new type of terrestrial sedimentation. • The ore-forming source materials were mainly derived from igneous and metamorphic rocks along craton margin. • Sufficient source materials, suitable environment, and post-deposition effect jointly led to massive bauxite formation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mineral composition of bauxite is affected by karstic topography. • Bauxite formation conditions range from oxidation to reduction. • Minerals deposited under different conditions are rich in microorganisms. • Microorganisms may promote the deposition of diaspore and boehmite. The metallogenic process of karstic bauxite in Central Yunnan Basin (CYB) remain obscure. Mineralogical, geochemical, and isotopic analyses of two profiles (profile A and B) in the Yuxi karstic bauxite deposit within CYB were conducted to explore the formation environment and metallogenic process of Early Permian bauxite. Profile A in the karstic uplifted area comprises clayey bauxite, claystone, and oolitic bauxite from the bottom to the top, while profile B in the karstic depression area contains oolitic bauxite, claystone, oolitic bauxite, and claystone. The oolitic bauxite mainly contains diaspore and chamosite, and a small amount of kaolinite and anatase, while the clayey bauxite primarily includes boehmite and kaolinite, and a small amount of goethite and anatase. The claystone is dominated by kaolinite and illite. The δ15N value of the oolitic bauxite in the top of profile B is close to zero (−0.8 ‰), indicating a reducing deposition environment. The δ13C org value of profile A is negatively shifted between clayey bauxite and oolitic bauxite, while that of profile B is not significantly changed between the lower oolitic bauxite and the upper oolitic bauxite. This indicates that REDOX changes in the formation of clayey bauxite and oolitic bauxite in profile A are obvious, while REDOX changes in profile B are not evident. Mineral assemblages and isotopic results suggest that the mineralization conditions of profile A experienced a transition from oxidizing and weakly alkaline to reducing and alkaline, while the mineralization conditions of profile B were always reducing and alkaline. The presence of large-scale microorganisms, negative δ34S values (−42.2 to −10.1 ‰), and framboidal and cuboidal goethite provide evidence for microbial involvement in the entire mineralization process. The above results confirm two metallogenetic events of Yuxi karstic bauxite deposit. In the early metallogenetic event, the water table was low and most of the karstic terrains were exposed except some karstic depressions. Abundant boehmite and kaolinite thus formed in the karstic uplifted areas forming the clayey bauxite, and diaspore and chamosite in the karstic depressions forming the oolitic bauxite. In the late metallogenetic event, with the rise of water table, all karstic terrains were submerged, and large-scale diaspore and chamosite were widely deposited, thus forming oolitic bauxite. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mineral paragenesis denote an alkaline and reducing metallogenic environment. • Microorganisms are widely involved in the formation of bauxite. • Contemporaneous volcanic activities are key for the bauxite formation. • Variations in pH and Eh along weathering profile lead to large-scale deposition. More than 1 billion tons of bauxite was formed on the karstic surface of the Permian Maokou Formation limestone in the western Guangxi area of Youjiang Basin (YB). However, its formation mechanism is still a matter of debate. In this study, the Taiping bauxite deposit, a representative large deposit in this area, was selected to carry out detailed mineral microanalysis and pyrite sulfur isotope analysis to further understand the metallogenic process of bauxite in this region. Taiping bauxite contains two layers: the lower bauxite layer and the upper claystone layer. Bauxite ore was mainly composed of diaspore, pyrite, anatase, kaolinite, minor pyrophyllite, parisite, and bastnäsite. Small amounts of detrital moissanite, native boron, and zircon were also found in the ore. The claystone was dominated by kaolinite, diaspore, pyrite, and chlorite. The occurrence of detrital zircon, moissanite, and native boron suggests that they were most likely derived from volcanic ash or lava, further confirming the contribution of volcanoes. The mineral assemblages of diaspore, pyrite, anatase, parisite, and bastnäsite in the bauxite ore indicate alkaline and reducing depositional environments. The extensive development of ovoid microorganisms and the significant negative δ34S values of pyrite (−42.80 ‰ to −12.54 ‰) indicate that microorganisms were involved in the formation of bauxite. This view was further confirmed by the wide development of fine pyrite (∼1 μm) cemented by cryptocrystalline diaspore in the ore. Extensive volcanism promotes the weathering of parent rocks, releasing Al3+, Fe3+, Ti4+, and REE3+ ions into the weathering system. Under strongly acidic and oxidizing conditions in the upper part of the weathering profile, these ions can migrate downward until they reach an alkaline and reducing environment on the carbonate surface. A large amount of diaspore, pyrite, anatase, parisite, and bastnäsite was formed near the karstic surface with the participation of microorganisms. Epigenetic modifications were also evident in the widespread development of kaolinite and pyrophyllite formed by the silicification of diaspore. [ABSTRACT FROM AUTHOR]